The present invention relates generally to card stock for photosensitive film mounts. In particular, the present invention is an aperture card (ie., a paper card with an opening in which a frame of film is mounted) suitable for archival storage.
Aperture cards such as that shown in FIG. 1 are commonly used for the handling and storage of photosensitive film such as microfilm frames bearing images of engineering drawings or other information. Aperture cards are typically of standard data processing punch card size, and include an aperture or hole sized to receive the microfilm frame. Strips of adhesive tape or other fasteners are used to mount the microfilm frame within the aperture or otherwise to the card.
Cameras, plotters, printers, duplicators and other types of equipment are used to process the information stored on aperture cards. Mechanical handling mechanisms drive the aperture cards within this equipment. To ensure consistent and reliable operation of the handling mechanisms, the aperture cards are manufactured to relatively strict specifications such as ANSI specification X311-1969. By way of example, some of the specifications and properties of conventional aperture cards commercially available from 3M of St. Paul, Minn., U.S.A., the assignee of the present invention, are as follows:
______________________________________ PROPERTY LIMITS ______________________________________ Thickness 0.007" .+-. 0.004 Burst (Mullen) 55 minimum Stiffness (Taber V-5) 17.0 minimum M.. 8.0 minimum C.D. Folding Endurance (MIT) 100 minimum M.D. 100 minimum C.D. Tearing Resistance (Elmendorf) 125 minimum M.D. 125 minimum C.D. Abrasion Loss (Taber) Top and bottom 50 maximum mg. los Soothness (Sheffield) Top and bottom 125 maximum Electrical resistance 40-250 Mohms Ash 2.0% maximum Coefficient of Friction 0.30 minimum 0.45 maximum Kinetic friction should not vary more than 25% from static friction Moisture content 4.5% minimum 6.5% maximu Expansion (20%-75% RH) 0.0184" maximum length 0.0228" maximum width Contraction (75%-20% RH) 0.0184" maximum width 0.0228" maximum width ______________________________________
Card stock from which the aperture cards are cut is manufactured on conventional papermaking machinery using standard formulation and papermaking techniques. The bulk of the formulation is a slurry of bleached hardwood and softwood chemical pulp, generally in a 60/40 to 80/20 hardwood to softwood ratio. Alum (Aluminum Sulfate) is added to the slurry for pH control, retention of additives such as internal sizing agents, and formation improvement. Retention and formation aids such as polyacrylamides and cationic starches can also be used. The stock is typically surface sized with starch as it is being manufactured, a step which can also have an effect on the coefficent of friction.
Information which must be retained for long periods of time is often recorded on microfilm frames mounted to aperture cards. Aperture cards used with "archival" film, that which has characteristics suitable for the preservation of records having permanent value, must meet the photographic activity test and alkali reserve test of the American National Standards Institute (ANSI) standard IT9.2-1988. Unfortunately, commercially available aperture cards of the type discussed above are somewhat acidic (pH 4.5-6.0) because of the alum used in the stock formulation, and do not meet this standard. Although paper stock with an alkali reserve is commercially available, this alkaline stock does not meet the photographic activity test and does not have the physical properties and specifications required of aperture cards.
It is evident that there is a need for "archival" aperture cards. To be suitable for archival use, the card stock must meet ANSI standard IT9.2-1988 and have the properties required of conventional aperture cards. To be commercially viable, any such archival card stock should be capable of being manufactured using conventional papermaking machinery.